Enteric coatings are widely used for various purposes including the protection of acid-labile medicaments from gastric acid and the protection of gastric mucosa from irritative or attacking medicaments. Known enteric coating compositions include cellulose compounds such as cellulose acetate phthalate, cellulose acetate trimellitate, hydroxypropyl methyl cellulose phthalate, hydroxypropyl methyl cellulose acetate succinate, and carboxymethyl ethyl cellulose; vinyl compounds such as polyvinyl alcohol acetate phthalate; and acrylic compounds such as methacrylic acid-ethyl acrylate copolymers. These bases are used in coating after the polymers are dissolved in organic solvents or aqueous latexes or water dispersions of the polymers are formed.
However, since these commercially available enteric polymers have a dissolution pH in the range of 5 to 7, they suffer from a decline of bioavailability. More particularly, when the polymer is applied to a medicament whose absorption site is limited to the upper portion of the small intestine, the enteric preparation (in the form of polymer-coated tablets or granules) will travel past the absorption site before it is sufficiently dissolved to release the medicament.
As a solution to this problem, JP-A 8-133989 and 8-301790 propose hydroxypropyl methyl cellulose acetate maleate and hydroxypropyl methyl cellulose trimellitate having a dissolution pH reduced to about 4, respectively. Because of the low dissolution pH, these polymers, apart from the concept of prior art enteric coating, are expected to find an application as an acid-resistant protective coating composition specialized for protecting medicaments from strong acidity. These polymers, however, are used as solutions in organic solvents which have the danger of explosion and fire, are hazardous to the safety and hygiene of workers, and cause environmental pollution as a result of release to the atmosphere. There is also a possibility that a minor amount of solvent be left in the coated solid preparation.